Estimation of Fetal Weight by MR Imaging to PREdict Neonatal MACROsomia (PREMACRO Study)

Macrosomia and growth restriction are important causes of perinatal morbidity, at or near to term. However, clear identification of 'at risk' foetuses is difficult and clinical estimates of fetal weight are poor. Historically, ultrasound has been used as a second line in such cases but the accuracy of this imaging modality in the mid- to late third trimester is also limited.

Estimated fetal weight (EFW) is an important part of the clinical assessment and is used to guide obstetric interventions, when a fetus is small or large for dates. When a diagnosis of intra-uterine growth restriction (IUGR) is made, the decision-making process is complex, particularly at very early gestations and involves multiple different factors, including maternal status, cardiotocography, liquor volume and dopplers. However, a large body of research is now available to assist with the management of both early and late-onset intrauterine growth restriction (IUGR) but there is a paucity of evidence to guide clinical practice, once macrosomia has been diagnosed, therefore the EFW is frequently the single most important component guiding interventions, such as induction of labour or Caesarean section.

Fetal macrosomia is associated with a higher incidence of perinatal morbidity, including shoulder dystocia and brachial plexus injury in the fetus and anal sphincter tears, uterine atony and haemorrhage in the mother. A recent multicentre randomised controlled trial appears to confirm the advantages of a policy of induction of labour for suspected macrosomia, demonstrating a clear reduction in the rates of shoulder dystocia and composite perinatal morbidity. However, some earlier but lower quality, observational studies have questioned the benefit of EFW made by ultrasonography in the last trimester, for suspected macrosomia, demonstrating that this practice can increase the risk of caesarean and instrumental delivery, without reducing perinatal morbidity.

Despite this conflicting data and a lack of evidence to support routine third trimester ultrasound, the absence of specific guidance, coupled with concerns regarding perinatal outcomes,mean that obstetricians will increasingly request an ultrasound at around 34-36 weeks gestation to identify foetuses above the 90th or below the 10th centiles. This practice will inevitably lead to increased and potentially harmful interventions based on relatively inaccurate data.

Due to the imprecision of ultrasound-derived EFW, particularly in cases of suspected macrosomia in the 3rd trimester, the investigators believe that these estimates should not be used to make important obstetric decisions regarding mode and timing of delivery and that a more accurate method of assessment could produce better outcomes by restricting interventions to those foetuses at greatest risk. Some publications have already demonstrated that magnetic resonance (MR) imaging derived-EFW close to delivery, is more accurate than ultrasound, with a mean percentage error superior to that of ultrasound and a recent meta-analyses has confirmed this promising accuracy.

The goal of the present study is thus to compare the performance of magentic resonance imaging derived-EFW, versus ultrasound derived-EFW at 36 weeks of gestation, regarding the prediction of neonatal macrosomia.

Inclusion criteria

• Subjects is ≥ 18 years of age and able to provide a written informed consent.
• Subject is a pregnant woman carrying a live singleton fetus at the 36+0-36+6 weeks scan, with no major abnormalities appearing during prenatal imaging with no major abnormalities appearing during prenatal imaging potentially affecting the correct use of the Hadlock formula for US-EFW. Conditions such as congenital diaphragmatic hernia with decreased abdominal circumference could be underestimated by the Hadlock USEFW. Another example is a massive sacro-coccygial teratomas.
• Subject is planning a delivery at our maternity at the University Hospital Brugmann, in Brussels, Belgium.
• Subject is known not to have any contra-indication to undergo an MR imaging examination.

Exclusion criteria

• Subject is known to have a contra-indication to undergo an MR imaging examination such as: Carrying a pacemaker or a metallic cardiac valve, having metallic material inside the head, having metallic fragments inside the eye following an accident, having any type of implant including ear implant, having a hip prosthesis
• Subject presenting with painful regular uterine contractions or history of ruptured membranes.
• Subjects who are unconscious, severely ill, mentally handicapped or under the age of 18 years.
• If birth occurs before MR and US evaluation.
• If patients delivers outside our local maternity unit.
• If the neonate's weigh is not measured within 6 hours after birth for any reason, including the need for emergency care immediately after delivery